The present invention concerns improvements made to pumping by non-evaporable getter (NEG) to create a very high vacuum in a chamber defined by a metal wall capable of releasing gas at its surface.
In a dehydrateable metal system in which a very high vacuum is to be made (i.e. a vacuum of at least 10xe2x88x9210 torr, or even of an order of magnitude of 10xe2x88x9213 to 10xe2x88x9214 torr), the metal walls of the vacuum chamber constitute an inexhaustible source of gas. The hydrogen contained in the construction metal (for example stainless steel, copper, aluminum alloy) diffuses freely in the thickness of the metal and is released at the surface defining the chamber. Likewise, when the vacuum chamber walls are bombarded with particles (synchroton radiation, electrons or ions)xe2x80x94as is the case in particle acceleratorsxe2x80x94, the result is the expulsion also of heavier molecular species, such as CO, CO2, CH4, produced at the surface after dissociation of hydrocarbons, carbides and oxides.
The level of vacuum obtained in the chamber is therefore defined by the dynamic equilibrium between the degassing at the surface defining the chamber and the pumping speed of the pumps used. Obtaining a high vacuum implies both a high order of chamber surface cleanliness reducing gas emission and a high pumping speed. For the vacuum systems of particle accelerators the chambers of which are generally of small section, pumps must be brought closer to each other or else continuous pumping has to be used, so as to overcome the limitation of conductance.
In these conditions, in order to obtain as high a vacuum as possible, it is known for the vacuum produced by mechanical pumps to be supplemented by carrying out additional pumping with the help of a getter placed in the chamber: this material is capable of producing chemically stable compounds by reaction with gases present in a vacuum chamber (particularly H2, O2, CO, CO2, N2) and this reaction causes the disappearance of the molecular species concerned, which equates to a pumping effect.
In order for the desired chemical reaction to occur effectively, it is necessary for the getter surface to be clean, i.e. free from any passivation coating formed during the exposure of the getter to the ambient air. This passivation coating may particularly be eliminated by diffusing the surface gases (O2 mainly) within the getter by heating (a getter activation process which is then designated as a non-evaporable getter: NEG). Non-evaporable getters have it the advantage of being able to be made in the form of a strip which can then be placed all along the vacuum chamber so that the result is a distributed pumping effect.
However, whatever pumping process is used, and despite the effectiveness of the distributed pumping made possible by the use of a non-evaporable getter, the level of vacuum capable of being obtained in the chamber remains defined by the dynamic equilibrium between the pumping speed (whatever means are used) and the speed of degassing from the metal surface of the chamber (whatever its cause); in other words for a given pumping speed, the level of vacuum remains dependent on the degassing rate in the chamber.
Document EP-A-0 426 277 describes a vacuum chamber arrangement for a particle accelerator, in which the wall inner surface is covered with a coating of getter material.
However, when the chamber is constituted by a metal foil shaped by bending, rolling, folding etc., the coating of getter material is deposited on the plane metal foil, before its shaping: during this shaping operation of the metal foil, the getter coating runs a very high risk of being damaged, or even torn off in places.
Likewise, when the chamber is defined by several assembled (for example bolted) parts, the getter material is deposited on each part individually before they are assembled. In this case, only the largest parts are treated, whereas the smaller parts are not: in addition, in this case too the getter coating runs a very high risk of being damaged during the assembly process; in the final analysis, the getter coating does not uniformly cover the whole inner surface of the chamber.
Lastly, in view of the fact that only one face of the metal foil or of the individual parts is coated with getter material, it is not possible for the coating to be formed by using a vacuum deposition process (for example cathode sputtering), the only one able to lead to the formation of a thin coating. As a consequence, as it is deposited by using a different technique, the getter coating is a thick coating. As a result, the effectiveness of this getter coating is inferior.
Document DE-Al-28 14 389 describes a process for reducing the residual gas density in a high vacuum chamber. To this end a getter material is activated by a plasma discharge; the surface obtained is then freed of its oxygen and has low degassing under irradiation. However, carbon has no getter action on the H21 CO, CO2 substances which are the residual gases present in an ultra-vacuum system once the water has been eliminated.
In these conditions, the getter used in this known process cannot be reactivated by simple vacuum heating: it is not a non-evaporable getter. Moreover, although the substance mentioned may be called a getter, it is certainly not able to provide a getter action in an ultra-vacuum metal chamber such as the chamber of a particle accelerator.
The object of the invention is thus to propose an improved solution which allows this problem to be solved and which, because of the degassing rate occurring in the chamber, notably increases the effectiveness of the pumping means used and leads to an improvement of several orders of magnitude in the level of vacuum capable of being created in the chamber.